1. Field of the Invention
The present invention relates to aqueous resin compositions which are effective binders for aqueous coatings, production methods therefor, and aqueous coatings using these aqueous resin compositions. More specifically, the present invention relates to aqueous resins which have excellent adhesion with respect to polyolefin resins such as polyethylene, polypropylene, ethylene-propylene copolymers, and ethylene-propylene-diene copolymers, which are effective as binders for aqueous resins which can form films with excellent film properties such as weather resistance and water resistance, production methods therefor, and aqueous coatings using these aqueous resin compositions.
2. Description of the Related Art
Conventionally, coatings for molded articles, sheets, films and the like which have polyolefin resins as the base materials are usually formed by blending various resins to chlorinated polyolefins or by denaturing chlorinated polyolefins with various resins and using these as the coating binders. However, since these chlorinated polyolefin type resins dissolve only in organic solvents such as toluene, xylene and the like, the resulting coatings contain large amounts of organic solvents and are known as organic solvent type coatings which have many problems in terms of health, environmental contamination, and hazardousness.
Therefore, in order to improve on the above-mentioned problems of organic solvent type coatings, the study of polyolefin resin coatings which do not include organic solvents has been advanced. In one example, chlorinated polyolefin resins are dispersed in water and emulsified for use as a coating binder, but these chlorinated polyolefin resin emulsions have poor adhesion to polyolefin resins, are difficult to disperse in water in microparticulate form, and have a complicated production process. Hence, these have not yet been put into actual practice.
As a specific example, Japanese Patent Application, First Publication No. 5-59188 proposes a method wherein chlorinated polyolefin resins are dissolved into an organic solvent and this solution is forcedly dispersed into an aqueous medium, after which the solvent is removed to obtain an aqueous compound. However, with this method, the viscosity of the chlorinated polyolefin resin solution itself makes it difficult to disperse in an aqueous medium in microparticulate form, and the surface smoothness of the resulting coating film is poor due to insufficient long-term stability of the dispersion fluid itself. Hence, while it can be used as an undercoated coating such as a primer, a base coat or a sealer, it is not suitable as a top-coated coating, and it also raises costs due to the need for steps of dissolving the chlorinated polyolefin resins and removing the solvent.
Additionally, Japanese Patent Application, First Publication No. 5-209006 proposes a method wherein chlorinated polyolefin resins are dissolved into an acrylic monomer and this solution is forcedly dispersed into an aqueous medium, after which a polymerization process is applied to obtain an aqueous compound. However, as with the above method, this method involves dispersing a chlorinated polyolefin resin into a solution, making microparticulate dispersion into water difficult. Additionally, chlorinated polyolefin resins are such that as the degree of chlorination decreases, their compatibility with acrylic monomers generally decreases and their compatibility with acrylic monomers produced by polymerization also decreases, so that the chlorinated polyolefin resins and acrylic monomers separate inside droplets dispersed within the aqueous medium. In some cases, the acrylic monomers separate into layers during the polymerization process and cover the surfaces of the produced particles, so that the adhesion to polyolefin resins decreases dramatically and the state of mixture between the chlorinated polyolefin resins and the acrylic monomers within each particle become nonuniform so as not to allow for sufficient denaturing effects from the acrylic monomers.
Recently, the need for coatings which are capable of being coated onto non-surface-treated polyolefin resin base materials in a single coat has increased due to the consideration of recycling of plastics and the prevention of ozone depletion. Consequently, polyolefin resins with very low chlorinations of 25% and under have garnered interest as coating binders, but these low-chlorination polyolefin resins are difficult to dissolve in (meth)acrylic monomers. Therefore, it is very hard to obtain aqueous compounds using the method of Japanese Patent Application, First Publication No. 5-209006.
Furthermore, Japanese Patent Application, First Publication No. 7-300570 proposes a method wherein chlorinated polyolefin resins are dissolved in an organic solvent with a solubility of 18.5% or less with respect to water, forcedly dispersed in an aqueous medium, emulsified, then added to an acrylic monomer pre-emulsion and finally polymerized to obtain an aqueous compound. However, since highly chlorinated polyolefin resins having chlorinations of 50-71% are used in this method, adhesion to the polyolefin resin base material absolutely fails to occur, and the method can only be applied to special uses for concrete base materials. Additionally, the particles of the acrylic monomer pre-emulsion often do not sufficiently mix uniformly with the particles of the chlorinated polyolefin resin emulsion and instead form separate polymers, so that the resulting resin composition is nothing more than a simple blend between the chlorinated polyolefin resin emulsion and the acrylic monomer emulsion. Consequently, the adhesion to the polyolefin resins further decreases while the denaturing effect due to the acrylic polymers is not very significant, and when a coating is formed by mixing with another type of emulsion, the stability in storage is worsened.